Now you're cooking with gas....you're starting to get into the art or science of 80's card reading. Probably an art because it's not really an exact science. And there's the judgment call when you're not 100% sure of what year you've got (which is generally the case.)
If it were me, I would keep Gibson for a while until you can get more information that might help you work out what year you've got. Gibby is one of the best value players in the game, IMHO, and so I would not drop him lightly. You're only 6 games into the season, so there's plenty of time to go before you have to make a decision. On the minus side for Gibson is the .214 BA. On the plus side are the five walks and the double and homer. His walk frequency is higher in 86 as is his homer frequency. That might just point to the better year. What I would do would be to start platooning him when he's back from injury--he could use a platoon in either year-- and see how he hits. If he keeps up his homer rate and draws a lot of walks, it's probably 86. If the walks and homers drop off, it might be 83. If 83, then dump him. But I wouldn't drop him without giving it a bit more time. 86 is a very good year in Tiger, where he should hit a lot of home runs.
Just my 2 cents.